The present invention relates to vehicle propulsion control systems and in particular to systems for the automatic control of the propulsion of vehicles driven by an engine connected through a gearbox to the driving wheels.
Propulsion control systems are well known in which manual gear changing is eliminated by the use of an automatic gear box incorporating a hydraulic torque converter, in which gear ratios are selected by reference to the road speed of the vehicle and the extent of depression of the accelerator pedal, so that the effective gear ratio is largely selected on the basis of the load on the engine. Gear boxes have also been proposed providing continuously variable ratios instead of discretely selectable ratios. Such systems are usually essentially mechanical in nature; and on account of losses in the torque converter, they require an increase in fuel consumption typically of the order of 5% over a normally driven manually controlled vehicle. It has also been proposed to employ an electronic data processing arrangement for similar control purposes incorporating stored data tables to specify appropriate gear ratios by reference to road speed and throttle position factors. To facilitate the selection of gear ratios in this way it is usual to employ a so-called automatic gearbox in which a number of predetermined gear ratios are provided by sun and planet gear configurations, the selection of a particular one of these ratios being accomplished by the application of a brake to the appropriate configuration. Such brakes are usually controlled by electrically tripped hydraulic selectors.
Whereas in the case of a manually controlled vehicle the driver usually has pedals to control the throttle (the accelerator pedal), the brakes and the clutch, the systems outlined above frequently require only two pedals, accelerator and brake, the clutch being automatically engaged when the accelerator pedal is depressed. A variety of automatic clutches have been proposed for this purpose, some relying on mechanical principles and others on electro-magnetic phenomena for their operation. However, in the previously proposed systems, the application of the automatic control arrangement is confined to provision of means for the generation only of positive thrust to drive the vehicle. Thus, braking, which arises as a consequence of the driver's demand for negative thrust, is directly under the control of the driver and is not communicated to the control system. Hence, such a control system has no means for assessing the driver's desire for negative thrust and consequently incurs avoidable losses by applying engine braking when zero positive thrust is requested unaccompanied by a demand for negative thrust.
Thus, there is, inherent in the control systems so far reviewed, a measure of uncertainty in the control input in distinguishing between the cessation of positive thrust requirements and an explicit demand on the part of the driver for generation of a negative thrust requirement. It is an object of the present invention, therefore, to provide an efficient control system able to distinguish between these three conditions, namely, positive thrust, negative thrust and neither, and also to arrange the system to generate negative, as well as positive thrust.